The present invention relates to a surface electrode structure on a ceramic multi-layer substrate and a process for producing surface electrodes on the substrate, which can be applied to the production of high-frequency modular parts having soldered surface-mounted parts and flip chip mounted surface-acoustic-wave devices on a ceramic multi-layer substrate and which enable consistent mounting of individual parts to provide higher reliability.
The market always demands smaller electronic equipment and it also demands reduction in the size and weight of the parts used. This tendency is noticeable in high-frequency equipment typified by cellular phones and is particularly pronounced in the parts used. In high-frequency equipment, there has been an increase in the parts mounting density to meet the demand for smaller size and lighter weight. Device mounting substrates are not an exception and in order to meet the demand for smaller size, the substrate having a single conductor layer is mostly replaced by the multi-layer substrate.
The ceramic multi-layer substrate has the insulating layers formed of electrically insulating ceramics and has the conductor layers formed of silver or the like. Compared to the conventional resin multi-layer substrate, the ceramic multi-layer substrate has many advantages including low loss at high frequencies, good heat conduction, as well as high dimensional precision and reliability.
The ceramic multi-layer substrate has a further advantage in that by forming a coil of the internal conductor or parallel plates of it, an inductance or a capacitance can be provided internally. What is more, the low loss and high dimensional precision features enable internal formation of high-Q and small-tolerance devices.
These features are deliberately utilized in cellular phones and other high-frequency circuits as a module or a device assembly of various parts that are mounted on the surface and which also have high characteristics, as well as meet the demand for smaller size.
In the high-frequency module, circuits are grouped by function, so compared to the conventional technique of forming circuits by mounting discrete parts individually, it allows for a simpler equipment structure and can provide equipment of better reliability and characteristics. Speaking further of the conventional discrete parts, their characteristics are combined to perform the intended functions and this results in a complicated design. In modularization, the specifications for the characteristics of each module are predetermined, so equipment design can be structurized and completed in a shorter period with reduced labor.
FIG. 6 is a block diagram for the GSM dual-band cellular phone which is used at the largest number of terminals in the world. In the figure, ANT designates an antenna for transmitting and receiving radio waves, DPX a diplexer (two-frequency switching filter) as a filter to separate two frequencies, T/R SW a transmission/receiving conversion switch as a means of switching between transmission and receiving of radio waves, LPF a low-pass filter as a filter to suppress harmonics at the transmission stage, and BPF a bandpass filter at the receiving stage.
In the illustrated circuit of cellular phone, modularization is realized for several functions and, in a typical case, devices are actually mounted on a multi-layer substrate in the antenna/switch section.
FIG. 7 shows an exemplary module for the antenna/switch section. In the figure, numeral 10 designates a ceramic multi-layer substrate having an inductor portion 11 and a capacitor portion 12 in the interior, as well as an external electrode 13. Chip components 15 such as diodes working as switching elements and resistors are mounted on the ceramic multi-layer substrate 10 and a shield case 16 is provided to cover up the top of the ceramic multi-layer substrate. Note that the module shown in FIG. 7 does not include surface-acoustic-wave devices (hereunder referred to as SAW devices) or they are mounted as packaged parts.
As of today, modularization has been realized in mono-functional devices such as power amplifiers and antenna/switch modules. If a broader range of functions are modularized, the advantages of modularization will be further obtained. Of course, modularization of devices including SAW devices is also important.
Conventional SAW devices have used so-called xe2x80x9cpackagedxe2x80x9d parts. Modularization can of course be realized by mounting packaged parts; however, as will be described later in the present invention, direct mounting of device chips on a substrate is believed to be more effective in realizing smaller and lower profiles, as well as lower cost.
The ceramic multi-layer substrate is characterized by its ability to contain inductance and capacitance as built-in parts to thereby allow for size reduction. On the other hand, lower profiles are difficult to realize. Therefore, common modules having packages mounted on a substrate cannot fully meet the ever increasing demand for lower profiles. In addition, packaged devices will occupy larger areas than the initial bare chips. Among the parts used, SAW devices are of the highest profile and occupy the wider area. Under these circumstances, it is desired that SAW chips be somehow mounted directly on the ceramic multi-layer substrate without using packages.
The production of SAW devices consists of two steps, one for fabricating SAW chips and the other for mounting and sealing them in package, and each step requires similar amounts of cost. If direct mounting on the ceramic multi-layer substrate is possible, inexpensive equipment can also be made in the absence of the step of mounting and sealing SAW devices in package.
As described above, it is desired for high-frequency modules that SAW devices be directly mounted as chips on the ceramic multi-layer substrate and that other parts be mounted by soldering.
To this end, the ceramic multi-layer substrate must be compatible with both the step of flip mounting SAW devices and the step of soldering other parts.
SAW devices are commonly bonded by gold-gold bump bonding with gold (Au) forming the topmost layer of the surface electrodes on the ceramic multi-layer substrate. In bonding by solder, the surfaces of lands on the substrate are commonly made of a tin or solder film, each of which is usually formed by plating.
The soldering process commonly comprises the steps of applying a paste of solder to the lands on the substrate surface, then placing the parts to be soldered, and performing a heat treatment such as reflowing to fix the parts. In this case, the flux in the paste of solder evaporates and the interface with the surface electrodes is activated to secure the wettability by the solder.
In the present invention, it is presupposed that SAW devices are mounted as exposed, so if they are first mounted, their characteristics will be greatly affected by the flux deposited in the subsequent step of soldering other parts. Hence, no method has yet been established that enables both the mounting of SAW devices in a bare state and the soldering of other parts as in the present invention.
The currently available small-size SAW device, as typically disclosed in Unexamined Published Japanese Patent Application (kokai) No. 10-79638/(1998), is fixed to a ceramic substrate or a resin substrate by a method called xe2x80x9cflip chipxe2x80x9d mounting. This is shown in FIG. 8, in which 20 designates the substrate and 30 a flip chip as the SAW device. Formed on the substrate 20 are electrodes 21 whose surface is made of gold (Au), and the flip chip 30 has gold stud bumps 31 formed on the principal surface having an SAW ladder-shaped electrode. With the SAW ladder-shaped electrode carrying principal surface facing down, the flip chip 30 is flip mounted by gold-gold bonding (face-down bonding).
This method would be effectively adopted in the present invention to mount SAW devices but it must satisfy the condition that no problem occurs if the SAW devices are mounted together with soldered parts. Unlike in the case of modularizing SAW devices alone, assembling a composite module with other parts will increase the thickness of the ceramic multi-layer substrate. In this case, the bonded areas will be subject to a greater stress in ordinary packaged devices.
Unexamined Published Japanese Patent Application (kokai) No. 6-97315/(1994) discloses a prior art case of mounting and sealing SAW devices together with other circuit components. In this prior art case, SAW devices are fixed, with face up, to the resin substrate and electrical connection is established by wire bonding; this is clearly different from the present invention which assumes the flip chip mounting of SAW devices on the ceramic multi-layer substrate. The present invention further achieves size reduction by adopting flip chip mounting. Another difference is that the effect of the thermal expansion difference from the substrate can be reduced by using the xe2x80x9cflip chipxe2x80x9d morphology. According to Unexamined Published Japanese Patent Application (kokai) No. 6-97315/(1994), supra, the ceramic substrate has a thermal expansion difference and is therefore problematic but in the present invention, the effect at issue is extremely attenuated. In particular, the temperature coefficient of SAW device and the thermal expansion difference cancel each other and if one compares the temperature characteristics of center frequency for the flip chips mounted on the resin substrate and the ceramic substrate, the result is better with the ceramic substrate as FIG. 5 shows.
It first appears that Unexamined Published Japanese Patent Application (kokai) No. 6-97315/(1994), supra, teaches the mixed mounting of SAW devices with other passive components but it does not presuppose the mixed mounting with parts to be soldered as in the present invention. For the sealing purpose, solder is used in the patent and for that matter, simultaneous heating is proposed to avoid contamination with the flux. In other words, it is suggested that mixed mounting with soldered parts is extremely difficult to realize.
The above statements dictate the attainment of two objects. One is to provide an electrode structure that can establish connection in both the soldering step and the step of mounting SAW flip chips, and the other is to develop a process flow that will not cause any effect on either step. Speaking of the process flow, soldered parts are first mounted and then the flux on the surfaces where SAW devices are to be mounted is removed by dry etching or the like and this enables the mounting of the SAW devices.
The present invention is particularly directed to the first object, i.e. providing an improved electrode structure. To attain this object, the following problems have to be solved.
(1) Providing the topmost layer which is made of such a material in such a structure that it is suitable for both gold-gold bonding and bonding by solder.
(2) Permitting consistent gold-gold bonding by forming electrodes that have small differences in height and which have a smaller number of surface asperities.
The present invention has been accomplished under these circumstances and has as objects providing a surface electrode structure on a ceramic multi-layer substrate and a process for producing surface electrodes on the substrate that are suitable for both mounting of SAW devices by gold-gold bonding and mounting of other parts by soldering and which enable consistent mounting of individual parts using a ceramic multi-layer substrate to provide higher reliability.
Other objects and novel features of the present invention will become apparent in the following description of modes of carrying out the invention.
To attain the above-stated objects of the invention, according to first aspect of the present invention, it is provided a surface electrode structure on a ceramic multi-layer substrate having surface SAW device mounting surface electrodes for mounting surface-acoustic-wave devices by gold-gold bonding and soldered parts mounting surface electrodes, in that the lowermost layer is made of a sintered silver conductor which is partly buried in the ceramic multi-layer substrate, an intermediate layer made of a nickel or a nickel alloy layer, and a topmost layer made of a gold layer.
Further, according to the second aspect of the invention, the intermediate layer of the nickel or nickel alloy layer has a thickness of 1 xcexcm-10 xcexcm and that the topmost layer of the gold layer has a thickness of 0.3 xcexcm-3 xcexcm.
Further, according to third aspect of the invention, the sintered silver conductor has a thickness of 10 xcexcm-40 xcexcm and that a depth of said sintered silver conductor by which it is buried in said ceramic multi-layer substrate is 60%-95% of the thickness of said sintered silver conductor.
Furthermore, according to the fourth aspect of the invention, the SAW device mounting surface electrodes have a gap in height of no more than 3 xcexcm.
Moreover, according to the fifth aspect of the invention, it is provided a process for producing surface electrodes on a ceramic multi-layer substrate having surface SAW device mounting surface electrodes for mounting surface-acoustic-wave devices by gold-gold bonding and soldered parts mounting surface electrodes, the process comprising the steps of: pressing a unbaked ceramic multi-layer substrate coated with a paste of silver conductor and then baking the ceramic multi-layer substrate to form the lowermost layer of sintered silver conductor which is partly buried in the ceramic multi-layer substrate; forming a nickel or a nickel alloy layer as an intermediate layer by electroless plating; and forming a gold layer as the topmost layer by electroplating or electroless plating.
(1) To meet the need for providing the topmost layer which is made of such a material in such a structure that it is suitable for both gold-gold bonding and bonding by solder, the topmost layer of the surface electrodes may be formed of gold. Note that gold having a purity of at least 99.99% is desirably used in gold-gold bonding in order to provide better adhesion upon application of ultrasonic wave. This pure gold has high wettability with solder and is also suitable as surface electrodes for soldered parts. However, the pure gold is soft and is so susceptible to erosion by molten solder that it sometimes fails to maintain the electrode shape.
Turning to the ceramic multi-layer substrate, sintered silver is commonly used as the surface conductor on it. To ensure good adhesion to the ceramic material, a small amount of glass is added to the sintered silver. The sintered silver was initially a powder from which the particles have been grown, so it inevitably has low surface smoothness. It is also poor in bondability to gold and wettability with solder, which are two requirements to be met by the electrode structure the invention intends to provide.
One may think that these problems can be easily solved by adopting a dual structure consisting of the sintered silver conductor and the gold layer but the adhesion between silver and gold is poor and depending on conditions, silver is also susceptible to erosion by solder and the asperities in the underlying layer cannot be absorbed by gold alone, thus making it difficult to perform consistent bonding of gold bumps. Because these and other problems remain, the dual structure is not a practically feasible approach.
These problems can be solved by sandwiching a suitable thickness of nickel layer or nickel alloy layer between silver and gold. A nickel layer or a nickel alloy layer can be deposited in a fairly large thickness by, for example, plating and adheres very well to both silver and gold. In addition, they are not eroded by solder and can absorb asperities in the surface of the sintered silver conductor.
Great improvement can be achieved by pressing or otherwise planarizing the sintered silver conductor before sintering. If the sintered silver conductor is partly buried in the ceramic sinter by pressing, the adhesion between the ceramic substrate and the sintered silver conductor can be improved.
(2) As for the need to reduce the difference in height between surface electrodes on the substrate in order to perform consistent gold-gold bonding, the following can be said. To form the sintered silver conductor, a paste of silver is usually formed on the ceramic multi-layer substrate by screen printing. In this case, the coating weight of the paste varies with the size of the patterns provided on the screen and it tends to become thicker as the pattern size increases. If there is a difference in height between surface electrodes, defective bonding will occur between the gold bumps on the SAW device and the surface electrodes.
This problem can most effectively be dealt with by planarization of the initial sintered silver conductors and the difference in height that remains after sintering can be absorbed most noticeably as will be described later. The difference in height also tends to increase by plating.
In order to cover asperities in the surface of the silver, a nickel plate of about 5 xcexcm is sometimes deposited; in this case, the thickness of plate deposit has to be prevented from varying between patterns. To this end, electroless plating is particularly suitable.
Speaking now of gold, it is deposited in small thickness, so there is no particular problem if a specified film thickness is ensured; nevertheless, deposition of a thin film will lead to cost saving and to this end, electroless plating is again preferred.
According to the present invention, all of these problems can effectively be solved.